Filling devices of this kind are used in particular to dispense small dosage quantities which are needed for example in the pharmaceutical field. The receiving containers are often placed on a balance in order to weigh the mass of the substance delivered out of the dosage-dispensing device, so that the substance can subsequently be further processed in accordance with given specifications.
The substance of which a measured dose is to be dispensed is held for example in a source container or reservoir which is equipped with a dosage-dispensing head. The aim is now to deliver the dosage substance through an aperture of the dosage-dispensing device in such a way that at the end of the filling process a predetermined target mass of the substance is present in the receiving container. The important point is that the mass that is actually present in the receiving container should match the predetermined target mass as accurately as possible and that the target mass is exactly defined. It is further important that the filling process can be performed in the shortest time possible.
The known state of the art offers dosage-dispensing methods that are based on a volumetric measurement of the dispensed substance. For a substance of density ρ, a variable aperture cross-section A of the valve and a resultant delivery flow velocity u of the substance, one obtains the mass mz of the substance in the receiving container as:
                              m          z                =                ⁢                              ∫                          t              auf                                      t              zu                                ⁢                                                    m                .                            ⁡                              (                t                )                                      ⁢                                                  ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              auf                                      t              zu                                ⁢                      ρ            ⁢                                                  ⁢                                          V                .                            ⁡                              (                t                )                                      ⁢                                                  ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              auf                                      t              zu                                ⁢                                    ρ              ⁡                              (                Au                )                                      ⁢                          (              t              )                        ⁢                                                  ⁢                          ⅆ              t                                                              =                ⁢                              ∫                          t              auf                                      t              zu                                ⁢                      ρ            ⁢                                                  ⁢                          A              ⁡                              (                t                )                                      ⁢                          u              ⁢                                                          (                              A                ,                h                ,                d                ,                …                            ⁢                                                          )                        ⁢                          ⅆ              t                                          
The delivery flow velocity u in particular is subject to many influence factors such as for example the aperture area A of the valve, the static pressure resulting from the fill level height h of the substance in the reservoir, and the rheological properties of the substance such as for example the powder grain size d. The rheological properties in particular are often very complex and subject to influence factors that are known only with limited accuracy. For example, the delayed flow which occurs in a Bingham substance at the beginning of the flow process is difficult to take into account. Particularly in the dosage-filling of pulverous substances, factors such as grain size, moisture content and surface properties of the individual grains are of major importance.
A method of optimizing the accuracy of the target mass delivered from a reservoir into a receiving container during a dosage-filling process is disclosed in U.S. Pat. No. 6,380,495 B1. In the method according to this reference, a valve which is arranged between the reservoir and the receiving container is first held open for a certain time period at its maximum aperture and then abruptly closed. During the fill process, the weight of the substance in the receiving container is monitored with a balance. Inaccuracies occur in this case because a certain amount of time is needed for the closing of the valve and also because there is material in free fall between the valve and the receiving container during the filling process. As a result, the amount of mass indicated by the balance at the time of the abrupt closing of the valve is less than the final amount of mass which is present in the receiving container after the end of the filling process. This error is determined through a recursive method of least squares and corrected in a subsequent iteration of the fill cycle. A problem which presents itself here is the abrupt closing of the valve. During the abrupt closing, the substance being dispensed is exposed to additional forces. Especially when sensitive substances are being dispensed, such as for example fine chemicals or pharmaceutical substances, it is important that the substance is handled as gently as possible and exposed to as little stress as possible. Otherwise, there could be an undesirable change in the substance properties, i.e. the substance could get damaged. The abrupt closing could also lead to a compaction of the material. This compaction can change the material properties and thus the flow properties of the material, which would negatively affect the reproducibility of the filling process. A further problem lies in the fact that in the first fill cycle, there are no data available from preceding fill cycles. Accordingly, it is impossible to perform a correction for the first fill cycle, which may cause an error in the resultant fill quantity.
It is therefore an object to provide a method and a device to fill receiving containers with a predetermined target quantity of a free-flowing substance from a reservoir in a way that causes the least possible amount of stress on the substance to be delivered and provides the best possible accuracy.